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PLANT MOVEMENTS Movement is not usually associated with and least of all with trees or shrubs. If we think of movement in the narrow sense of an entire organism translocating, then we find very few instances in world. Perhaps, the best-known cases are when a unicellular alga such as Chlamydomonas moves towards light or when spermatozoids move towards the egg cell by means of flagella. Many locomotion’s are, however, auto­matic and do not depend on any external stimulus such as shown by plasmodesmata of Myxomycetes or amoeboid movements which are really creeping movements due to some internal stimulus. Plants possess irritability and execute movements in response to a variety of external stimuli which act on protoplasm of the cell. Such movements can only be induced because the stimuli act on an excitable protoplasm.

I. Paratonic movements of growth or tropical movements or : When growth movements occur in response to an external stimulus which is unidirectional, they are called as tropical movements and the phenomenon of such a movement is called as . Depending upon the nature of the unidirectional external stimulus the tropical movements are of many types. a) Geotropic movements or geotropism (): The tropical movements which take place in response to the gravity stimulus are called as geotropic movements and this phenomenon as geotropism.

Dr Ashwathanarayana R, Guest Faculty, Dept. of , SSCASCW, Tumkur

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The primary grow down into the soil and are positively geotropic. The secondary roots growing at rights angles to the force of gravity are called as Diageo tropic. While those growing at some intermediate angle (between 0° and 90° to the vertical) are said to be plagiogeotropic (plagiogravitropic). On the other hand, the primary stems are negatively geotropic.

The effect of the unilateral stimulus of gravity causes unequal distribution of growth hormone auxin in the tip i.e., more auxin concentrates on the lower side than on the upper side. This in turn results in more growth on the upper side and less growth on lower side, and ultimately a positive geotropic curvature is observed.

b) Phototropic movements or : The tropical movements which occur in re•sponse to an external unilateral light stimulus are called as phototropic movements. These movements are commonly found in young stem tips which curve towards the unilateral light stimu•lus and thus, are called as positively phototropic. This can be observed very easily by placing a potted plant in a room near an open window. After a few hours, the stem will be seen bending to•wards the window, the latter being the unilateral source of light. The roots in some plants also exhibit phototropic movements but they are negatively phototropic. When the stem tip receives uniform light all around, the concentration of the growth hormone auxin also remains uniform Dr Ashwathanarayana R, Guest Faculty, Dept. of Botany, SSCASCW, Tumkur

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in the tip. But when the tip receives unilateral light, the conc. of auxin be•comes more in the shaded side than in the lighted side. Consequently, the higher conc. of auxin in the shaded side causes that side to grow more result•ing ultimately in a positive phototropic curvature.

c) Thigmotropic or haptotropic movements: These movements take place in response to a touch or contact stimulus and are very common in plants which climb by tendrils. In such plants e.g., Passiflora, the tip of the tendril in the beginning moves freely in the air. But, as soon as it comes in contact with a solid object which may provide it support (i.e., it gets the contact stimulus), it twines round the object so that the plant may climb upward. The twining of the tendril around the support is due to less growth on that side of the tendril which is in contact with the support than the more growth on the free opposite side. d) Hydrotropic movement or : The tropical movements occurring in response to water stimulus are called as hydrotropic movements. These are commonly found in young roots. It can be demonstrated by the following simple experiment: seeds are sown in soil and some days after put a porous clay pot filled with water. After a few days, the radicles will be seen bending towards the moist pot direction.

Dr Ashwathanarayana R, Guest Faculty, Dept. of Botany, SSCASCW, Tumkur

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e) : Chemotropic movements occur in response to some chemical stimulus and are best exhibited by fungal hyphae and pollen tubes. f) and Aerotropism: These tropical move•ments are not very important. When they occur in response to temperature stimulus, they are called as thermotropic movements. In case the stimulus is air, they are called as aerotropic movements.

II. Paratonic movements of variation or : When growth movements occur in response to an external stimulus which is not unidirectional but diffused, they are called as nastic movements. These movements occur only in bifacial structures like , sepals, petals etc., and may be of many types: a. Nyctinastic movements (or sleep movements): In many plants the leaves and flowers ac•quire a particular but different position during day and at night. Such movements are called as nyctinastic movements or sleep movements. If these movements result in response to the presence or absence of light, they are called as photonastic movements e.g., Oxalis sp. where the flowers and leaves open in the morning and close at night. In other plants such as Crocus and Tulip the flowers open at higher temperatures. Such movements which occur in response to temperature stimulus are called as thermonastic movements. b. Seismonastic movements: These movements are best exhibited by sensitive plant such as touch-me-not plant () and occur in response to a touch or shock stimulus including shaking or wind, falling of rain drops, wounding by cutting and intense heating or burning. In this plant the leaves are bipinnately compound with a swollen pulvinus at the bases of each and similar but smaller pulvinules at the bases of each leaflet or pinna. If a terminal pinnule of a leaflet is touched or given a shock treatment, the stimulus passes downward to the pulvinule and all the pinnules of that leaflet get successively closed in pairs. Now the stimulus passes to the other pinnae or leaflets so that their pinnules also close down and finally it reaches the pulvinus resulting in

Dr Ashwathanarayana R, Guest Faculty, Dept. of Botany, SSCASCW, Tumkur

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drooping of whole of the leaf. Whole of this process is completed just in few seconds.

The pulvinus contains a number of specialised large thin walled parenchymatous cells called motor cells which undergo reversible changes in turgor in response to the stimulus. When stimulus reaches the pulvinus, the osmotic pressure of motor cells is decreased. Consequently, water is released from them into intercellular spaces and they suddenly collapse resulting in drooping down of the leaflets and the leaf. After the lapse of sometime, the leaf recovers from the shock or touch stimulus, the turgor of motor cells is restored and the leaflets and the leaf come in their normal erect position. c. Thigmonastic or haptonastic movements: The movements are found in the leaves of Drosera (Sundew) and Dionaea (Venus Fly Trap) and result in response to the touch stimulus of the insects. In Drosera, as soon as an insect sits on the leaf, the tentacles curve inward to trap the insect. Similarly, in Dionaea, the two halves of the leaf curve upward along the mid•rib. These parts of the leaves come to their normal position after the insect has been digested.

Drosera and Dionaea showing thigmonastic movement

Dr Ashwathanarayana R, Guest Faculty, Dept. of Botany, SSCASCW, Tumkur